test_shm_mq is an example of how to use dynamic shared memory and the shared memory message queue facilities to coordinate a user backend with the efforts of one or more background workers. It is not intended to do anything useful on its own; rather, it is a demonstration of how these facilities can be used, and a unit test of those facilities.

The function is this extension send the same message repeatedly through a loop of processes. The message payload, the size of the message queue through which it is sent, and the number of processes in the loop are configurable. At the end, the message may be verified to ensure that it has not been corrupted in transmission.

This function sends and receives messages synchronously. The user backend sends the provided message to the first background worker using a message queue of the given size. The first background worker sends the message to the second background worker, if the number of workers is greater than one, and so forth. Eventually, the last background worker sends the message back to the user backend. If the repeat count is greater than one, the user backend then sends the message back to the first worker. Once the message has been sent and received by all the coordinating processes a number of times equal to the repeat count, the user backend verifies that the message finally received matches the one originally sent and throws an error if not.

This function sends the same message multiple times, as specified by the repeat count, to the first background worker using a queue of the given size. These messages are then forwarded to each background worker in turn, in each case using a queue of the given size. Finally, the last background worker sends the messages back to the user backend. The user backend uses non-blocking sends and receives, so that it may begin receiving copies of the message before it has finished sending all copies of the message. The verify argument controls whether or not the received copies are checked against the message that was sent. (This takes nontrivial time so it may be useful to disable it for benchmarking purposes.)

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